1989 GMC Forward Control 5.7 K, Automatic, 3L80/M40FACTORY OEM SERVICE PROCEDURE

Fuel System Pressure

  1. Turn engine off and relieve fuel pressure. See FUEL SYSTEM PRESSURE RELIEF. Remove air cleaner and plug air cleaner (THERMAC) vacuum port (if equipped).
  2. Disconnect fuel line between throttle body and fuel filter. Install Fuel Pressure Gauge (J-29658A) and Adapter (J-2968A-85) between throttle body and in-line fuel filter. Start engine and observe fuel pressure reading.
  3. Fuel pressure should be 9-13 psi (.6-.9 kg/cm2 ). If not, see CHART A-6, FUEL SYSTEM PRESSURE TEST in the CCC TBI TESTING/CODES article in the COMPUTER ENG CONTROL section.
  4. After testing, relieve fuel pressure. Remove fuel pressure gauge and connect fuel line to fuel filter. Start vehicle and check for leaks. Remove plug from throttle body vacuum port (if equipped). Install air cleaner.
Factory Service Tools

Use the Manual With the Right Hardware

Pair factory procedures with proven DIY tools so the instructions are easier to execute.

Disclosure: Verified tool recommendations may earn an affiliate commission at no cost to you.

Shop Manual-Ready ToolsCompare Repair QuoteQuote Shield Pro

When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.

Rather have a pro handle this?

A certified mobile mechanic comes to your home or office. They bring the tools, you stay comfortable. No tow truck, no waiting room, no surprises.

Comes to youSame-day available12-month warranty
Get a Free Quote

No obligation. See pricing before you book.